How to Test for a Tire's Speed Rating

Tire speeds are tested by professionals with strict adherence to speed limits.

Many of us would like to drive a high performance vehicle. Due to their elevated status as the fastest cars, tire manufacturers use the speed capabilities of these vehicles to test their tire ratings. Tires are speed rated in laboratory settings and the resulting tire rating tells you the maximum speed your tires can withstand. Ratings range from L (75 mph) to ZR (over 150 mph).

Things You'll Need

• Test tire assembly
• Test wheel or metal drum

Instructions

1. Inflate the test tire to a pressure of 3psi less than the maximum stated on the tire's sidewall.

2. Press the test tire assembly up against the metal drum or test wheel. According to National Highway Traffic Safety Administration you should do this "with a load of 88% of the tire's maximum load rating as marked on the tire sidewall."

3. Run the tire for two hours at increasing speeds in ten minute increments of 6.2 mph intervals until you get to 50 mph. Then cool it to about 100 degrees Fahrenheit. After the tire has cooled make sure it is inflated to the pressure specified on the sidewall.

4. Test the tire at 75 mph for half an hour, then 80 mph for another half hour and then 85 mph for a further 30 minutes. Cool the tire for an hour and then deflate and remove it from the test wheel. Check it for the failure criteria outlined by the manufacturer.

Tips & Warnings

• Tire speed ratings are only valid for undamaged, unaltered tires that have not been overloaded or under inflated. Tire manufacturers will not vouch for a rating on an altered tire as they are not in control of any changes that may have been made to the tire.

References

• "Wayne's Wheels"; Speed ratings
• "National Highway Traffic Safety Administration
• "Tire Rack"; How to Read Speed Rating, Load Index & Service Descriptions